1. Field of the Invention
This invention is related to the design and manufacturing of the photodiode stack uses to produce photo MOS relay, also known as solid state relay or semiconductor relay.
2. Description of the Related Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or form part of common general knowledge in the field.
A photo MOS relay is an Opto semiconductor device functioning like an electromechanical relay. Depending on the type of output MOS device uses, a photo MOS relay can be made either normally open or normally close. FIG. 1 shows an electrical block diagram of a normally open photo MOS relay. The input of a photo MOS relay is an infrared light emitting diode (LED). The output side of a photo MOS relay consists of a photovoltaic circuit and two MOS transistors. The photovoltaic circuit consists of a photodiode stack, turn on and off control circuits which normally are all fabricated on the same silicon substrate. Wire bonding is performed between the photovoltaic circuit to the gate and source terminals of the two MOS transistors. The input side and the output side of a photo MOS relay is separated by an electrically insulated material which is transparent to infrared light. The input LED, the insulation and light transmission medium, the photovoltaic circuit and the two MOS transistors are encased in molded plastic or ceramic package to produce a photo MOS relay device. When the input LED is carrying current, it emits infrared light. The infrared light shins on the photodiode stack and generates a photo current in the photo diode and a rise in voltage between the terminals of the photodiode stack. The photodiode stack voltage applies to the gate and source terminals of the two MOS transistors. If the MOS transistor is an enhancement type, the MOS transistors will turn on to emulate a closing of a normally open switch. If the MOS transistor is a depletion type, the MOS transistor will turn off to emulate an opening of a normally close switch.
The voltage generates across the photodiode stack in the present of infrared light mainly depends on the number of photo diodes in the stack. In a typical photo MOS relay, the number photo diode in the photodiode stack range from 14 to more than 20 photo diodes. In the prior art, the photodiode stack, the turn on and off control circuit components are fabricated using oxide isolation technology. The use of oxide isolation technology is mentioned in numerous US and international patents, ie. U.S. Pat. Nos. 6,806,482, 5,151,602, 4,873,202, . . . FIG. 2 and FIG. 3 show the top and cross section views on a section of the photo diodes stack build with oxide isolation technology. Each diode is resided in its own oxide isolated pocket so there is no concern regarding junction to substrate leakage or issue with deep junction photo current generation. While the oxide isolation technology is effective in producing the photodiode stack, the major drawbacks are the higher cost of fabrication, longer fabrication cycle time and difficulty in locating wafer foundry as oxide isolation fabrication technology is a niche and specialize process which very few companies have. Since the turn on and off control circuits are on the same substrate as the photodiode stack, the circuit components of the turn on and off circuits are fabricated on the expensive oxide isolated substrate. Due to the high cost relates to the oxide isolated technology, higher degree of circuit integration to the photovoltaic circuit is not economically feasible. In U.S. Pat. No. 6,750,523, a mean of producing the photodiode stack using standard junction isolation Bipolar or CMOS technology is described. However, the described method uses the substrate as a conductive path and nest diffusions to form the photo diode which limits the maximum number of photo diodes to 3 to 4. Since the photo current produces in the photodiode is on the order of a couple micro ampere, 3 to 4 photo diodes in series will generate about 1.5V to 2V at room temperature when the LED is carrying current. The voltage generates by 3 or 4 photo diodes in series is not high enough for photo MOS relay application. As mentioned earlier, 14 or more diodes in series are needed for typical photo MOS relay design due to the output MOS's input threshold voltage plus the gate to source over drive voltage requirements to conduct the relay's rated load current. Also, the diode's forward voltage of the photodiode stack has a negative temperature coefficient of about −2 mV/degree C. so the photodiode stack voltage must remain high enough for photo MOS relay high temperature operation. A photo MOS relay has many advantages over the traditional electromechanical relay like higher reliability due to no moving parts, smaller physical size, no relay bounce, lower input operating current and longer operation life. However, the main disadvantage for photo MOS relay compares to electromechanical relay is higher product cost. A major contributor to the higher cost of a photo MOS relay is due to the photodiode stack is fabricated using oxide isolation technology. Thus, a method of generating photodiode stack using lower cost junction isolation technology is highly desirable for photo MOS relay production.